1. Technical Field
This invention relates generally to the measurement of the ratio of toner particle electrostatic charge to toner particle mass in electrostatographic reproduction apparatus such as copiers and/or printers.
2. Background Art
The electrostatographic process is controllable by means of various parameters such as primary voltage V.sub.0, exposure E.sub.0, development bias V.sub.b, and the concentration of toner particles in the development mixture. Another important parameter in the development of latent electrostatic images is the ratio of toner particle electrostatic charge-to-toner particle mass.
Several factors influence the charge-to-mass ratio, such as for example, relative humidity, toner concentration, chemical contamination, developer mixture aging, etc. Developer mixture aging can result from larger toner particles tending to develop easier than smaller particles, leading to a decrease in average toner particle size and greater charge-to-mass ratio.
Changes in charge-to-mass ratio results in a tendency toward decreased toner density for a given difference in charge between the toner particles and the electrostatic image. Accordingly, it is desirable to monitor the charge-to-mass ratio so that other parameters can be adjusted to compensate for changes in the ratio. For example, feedback control can be used to adjust, say, the brush or photoconductor bias in response to changes in the charge-to-mass ratio and development rate.
U.S. Pat. No. 4,026,643, which issued to Bergman on May 31, 1977, discloses apparatus for measuring the charge-to-mass ratio by combining a measurement of the difference between the electrostatic charge on a photoconductor before and after development (proportional to the charge) with a measurement of a difference in optical reflectance in the presence and in the absence of charged toner particles (related to the toner mass). This system has the disadvantage that it must be calibrated for the optical reflectance of each toner color; and probably for each batch of the same color toner. In such devices, optical reflectance has to be calibrated for the optical reflectance of each type (color, etc.) toner used, and probably for each batch of the same toner types.